power: supply: Use devm_mutex_init()

power: supply: Use devm_mutex_init()

Use devm_mutex_init() instead of hand-writing it.

This saves some LoC, improves readability and saves some space in the
generated .o file.

As an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  35803	   9352	    384	  45539	   b1e3	drivers/power/supply/rt9467-charger.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  34792	   9008	    384	  44184	   ac98	drivers/power/supply/rt9467-charger.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>